The present invention relates to a particle monitor, and more particularly to a particle monitor for monitoring in-situ particles or precursors thereof existing in a space above a wafer to detect in real time a spatial distribution as well as to an apparatus for removal of the particles from the space above the wafer on the basis of information about a real time spatial distribution of the particles to prevent the particles from adhering to the wafer.
Crystal defects of semiconductor due to particles may affect the quality and reliability of microelectronics devices. Particulate contamination control is one of the most important and major factors on the yield of modern large scale integrated circuits and packaging fabrication lines. For these reasons, tests for particles are carried out after some processes. It is possible to irradiate a laser beam onto a chip adhered with particles to cause laser beam scattering from which particles are detected. It is also possible to detect differences in particle images between adjacent two chips.
As the large scale integrated circuit has been scaled down, the difference in level of the surface of the chip is larger than the particle size, for which reason it is difficult to detect particles.
In place of the tests after the processes, it is possible to measure particles in the apparatus.
In the Japanese laid-open patent publication No. 3-39635, it is disclosed to use the laser beam scattering for particle detection, wherein even if the particle size is not more than 0.07 micrometers, an accurate detection of particles is possible. The particle detector is provided with the following elements. A light source is provided for emitting a laser beam. A flow cell is provided on an optical axis of the laser beam wherein a measured fluid flows through the flow cell. A pair of photo-detectors are provided on the optical axis of the laser beam and at opposite sides of the flow cell. A detector section is provided which are respectively connected to the photo-detectors. The detector section further has the following elements. Two recognition circuits are provided to be connected to the photo-detectors for detecting output signals from the photo-detectors to recognize output signals which exceed a predetermined threshold value. A judging circuit is provided which is connected to the two recognition circuits for fetching the output signals that exceed the predetermined threshold value from the recognition circuits to select only synchronous output signals and generate an output signal when a pair of the synchronous output signals could be detected. A counter circuit is also provided which is connected to the judging circuit for receiving the output signals from the judging circuit to count up the received output signals.
In the Japanese laid-open patent publication No. 4-52077, and Japanese laid-open utility model publication No. 3-39721, it is disclosed to use a technique wherein scattered lights are converted into electrical signals for display. This technique is also disclosed, for example, by Gary S. Selwyn in J. Vac. Sci. Technol. B9(6), November/December 1991, pp. 3487-3492 or by Y. Watanabe et al. in Appl. Phys. Lett. 61(13), 28 Sep. 1992, pp. 1510-1512.
Further, there is another technique wherein a polarized laser beam is irradiated toward particles to cause polarized laser beam scattering and variations in polarization between the incident laser beam and the scattered laser beam are measured. A laser beam emitted from a laser oscillator is then transmitted through a polarizer so that the laser beam is linearly polarized at an azimuth angle of 45 degrees from a horizontal plane. The polarized laser beam is transmitted into the processing equipment. The polarized laser beam is then scattered by particles in the processing equipment. The scattered polarized laser beam is then transmitted through a rotational analyzer into a photo-detector for obtaining output signals from the photo-detector. The intensity of the output signals from the photo-detector are measured in two cases when a quarter wavelength plate is provided in front of the photo-detector and when no quarter wavelength plate is provided in front of the photo-detector. As a result of the measurement of the intensity, it is possible to obtain signals modified at a rotation frequency of the rotational analyzer and varying over time. The modified signals are subjected to Fourier transformation to obtain Stokes parameters which are necessary to decide compositions of particles, number density and spatial distribution. This technique is disclosed for example by Hayashi et al. in Jap. J. Appl. Phys. Vol. 33 (1994) pp. L476-L478.
The first conventional technique disclosed in the Japanese laid-open patent publication No. 3-39635 is directed to an apparatus for measurement of particles in the fluid such as liquid and gaseous fluids by use of the laser beam scattering, wherein the measurement is made by introducing the measured fluid into the flow cell, for which reason it is difficult to take an interrelationship with particles into account.
The second conventional technique disclosed in the Japanese laid-open patent publication No. 4-352077 is directed to the conversion of the scattered light into image signals, wherein the image obtained when any smoke or dust is emitted is compared with another image obtained when no smoke or dust is emitted, so as to obtain a difference between the image so that it is possible to confirm emission of any smoke or any dust, but impossible to measure the density and the size of the smoke or the dust as well as distributions thereof. This means it difficult to find the mechanism for emission of the smoke or dust.
Also in the above conventional technique disclosed in the Japanese laid-open utility model publication No. 3-39721, the scattered light is collect by a photo-detector so that a plurality of laser emission diodes aligned display the size of particles and so that concentrations of particles are printed out by a printing machine. It is, however, difficult to find the mechanism for emission of particles which affect the yield of the wafer.
The other conventional technique disclosed by Gary S. Selwyn and by Watanabe is directed to measurement of particles in the space above the wafer in the processing equipment by use of the scattered light so that the scattered light is measured by a charge coupled device camera to obtain images of the scattered light. The obtained images are two-dimensional images wherein the scattered light from final particles of a small size of about several tens nanometers is not distinguishable from the scattered light from large size particles of the sub-micron or micron order sizes. The particles are in whole observed and imaged as a shining cloud, for which reason the obtained images are stored in a videotape recorder for subsequent calculations of the spatial distribution in size or diameter of the particles based upon the luminance. For these reasons, it is impossible to make a real-time measurement of the spatial distribution of the size of the particles. This means it difficult to follow rapidly variable states of the processing equipment. Namely, it is difficult to prevent wafer loss.
In order to measure the size and the density of the particles, the scattered light from a point in the space is measured. It is however impossible to measure all the movements and variations over time of the particles. This means it difficult to follow rapidly variable states of the processing equipment. Namely, it is difficult to prevent wafer loss.
The other conventional technique disclosed by Hayashi et al. uses the rotational analyzer for polarization analyze of the scattered light, wherein the measurements have to be made in both cases when the quarter wavelength plate is inserted on the optical axis and when no quarter wavelength plate is inserted thereon to obtain Stokes parameters necessary for presumptions of the diameters of the particles and number density thereof as well as refractive index.
At the present, single wafer processing equipment has been the main equipment for LSI manufacturing processes. Normally, it takes about 60-120 seconds to process a single wafer, for which reason it is necessary to do a continuous monitoring of generation of particles and provide in real-time the monitoring results to process engineers. Notwithstanding, the time of processing the single wafer, for example, about 60-120 seconds are too short for the rotational analyzing method to conduct continuous monitoring and real-time report to the process engineers because the rotational analyzing method is carried out by conducting two measurements in both cases when the quarter wavelength plate is inserted on the optical axis and when no quarter wavelength plate is inserted thereon to obtain Stokes parameters necessary for presumptions of the diameters of the particles and number density thereof as well as refractive index.
In the above circumstances, it had been required to develop a novel particle monitor for monitoring in-situ particles or precursors thereof existing in a space above a wafer to detect in real time a spatial distribution of particles as well as an apparatus for removal of the particles from the space above the wafer on the basis of information about a real time spatial distribution of the particles to prevent the particles from adhering to the wafer.